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Driving a Capacitive Load

+

_

THS402x

C

LOAD

1kW

Input

Output

50 W

20 W

S0278-01

Offset Nulling

_

+

THS402x

V

CC–

V

CC+

0.1 F

m

0.1 F

m

10kW

S0279-01

THS4021

THS4022

SLOS265C – SEPTEMBER 1999 – REVISED JULY 2007

Driving capacitive loads with high-performance amplifiers is not a problem as long as certain precautions are

taken. The first is to realize that the THS402x has been internally compensated to maximize its bandwidth and

slew-rate performance. When the amplifier is compensated in this manner, capacitive loading directly on the

output decreases the device phase margin, leading to high-frequency ringing or oscillations. Therefore, for

capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with the output of

the amplifier, as shown in Figure 33. A minimum value of 20

Ω should work well for most applications. For

example, in 75-

Ω transmission systems, setting the series resistor value to 75 Ω both isolates any capacitance

loading and provides the proper line impedance matching at the source end.

Figure 33. Driving a Capacitive Load

The THS402x has very low input offset voltage for a high-speed amplifier. However, if additional correction is

required, an offset nulling function has been provided on the THS4021. The input offset can be adjusted by

placing a potentiometer between terminals 1 and 8 of the device and tying the wiper to the negative supply. This

is shown in Figure 34.

Figure 34. Offset Nulling Schematic

12

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Product Folder Link(s): THS4021 THS4022